封装一个并发函数

[wuweijia]

// 并发请求
const urls = []
for (let i = 1; i <= 20; i++) {
	urls.push(`https://jsonplaceholder.typicode.com/todos/${i}`)
}

const fetchMultiple = (urls, max, callback) => {
	const len = urls.length
	const result = []
	let requestIndex = 0

	const request = (index) => {
		requestIndex++
		const url = urls[index];
		fetch(url).then(res => res.json()).then(data => {
			console.log(`请求 ${url} 完成`, data);
			// 保存结果 要与 url 的位置对应
			result[index] = data
		}).catch(err => {
			console.error(`请求 ${url} 失败`, err);
		}).finally(() => {
			// 判断是否所有请求都完成
			if (result.length === len) {
				callback(result)
			} else if (requestIndex < len) {
				request(requestIndex)
			}
		})
	}

	for (let index = 0; index < max; index++) {
		request(index)
	}
}

fetchMultiple(urls, 5, (result) => {
	console.log('所有请求完成', result);
})